Crossed-field Amplifiers (CFAs) are a class of vacuum microwave devices where an applied direct current (DC) electric field is oriented perpendicular to a constant magnetic field. Typically, CFAs have a magnetic field oriented in an axial direction and an electric field applied around a circumference of a cathode. Crossed-field amplifiers are used in many types of radars, in part due to their high efficiency and broad bandwidth.
The most common type of CFA consists of a slow-wave-circuit (SWC) that surrounds a cathode in a cylindrical geometry. These devices generally consist of an input coupler for receiving a radio frequency (RF) input wave, a cathode for emitting electrons, a slow wave circuit, an anode drift block, and an output coupler for transmitting the amplified RF wave. The SWC is usually of the forward wave type in which the electron beam interacts with a wave propagating in the same direction as the electron beam; however, backward wave circuits are also possible, and are referred to as Amplitrons.
In use, the RF wave first enters the SWC through an input coupler. The cathode of the CFA emits electrons as a result of primary (thermionic) or secondary emission, or both. Under the influence of the crossed electric and magnetic fields, the electrons emitted from the cathode rotate around the cathode and form a thin region of high electron density near the cathode surface, known as a hub. When the outer surface of the electric hub has about the same velocity as the RF wave, the rotating electrons give up potential energy to the wave. This causes amplification of the RF wave, also known as gain.
In reentrant beam CFAs, after the bunched electrons pass the output coupler, the SWC ends and the spent bunches drift toward the input coupler without the influence of external RF fields. Ideally, the electron bunches would completely diffuse into a uniform electron stream before reentering the input section of the CFA. But because the spent electrons are in the drift section for a short period of time, there are fluctuations in the electron density upon reentry into the input section. These fluctuations can produce spurious emissions, also referred to as spurious noise. The spurious noise can interfere with radars and a variety of other communication systems.
Accordingly, there is a need for improved cross-field amplifiers with reduced spurious emissions.